Cu-doped SnO2/rGO nanocomposites for ultrasensitive H2S detection at low temperature

Hydrogen sulfide (H2S) detection remains a significant concern and the sensitivity, selectivity, and detection limit must be balanced at low temperatures. Herein, we utilized a facile solvothermal method to prepare Cu-doped SnO2/rGO nanocomposites that have emerged as promising candidate materials for H2S sensors. Characterization of the Cu-SnO2/rGO was carried out to determine its surface morphology, chemical composition, and crystal defects. The optimal sensor response for 10 ppm H2S was ~1415.7 at 120 °C, which was over 320 times higher than that seen for pristine SnO2 CQDs (Ra/Rg = 4.4) at 280 °C. Moreover, the sensor material exhibited excellent selectivity, a superior linear working range (R2 = 0.991, 1–150 ppm), a fast response time (31 s to 2 ppm), and ppb-level H2S detection (Ra/Rg = 1.26 to 50 ppb) at 120 °C. In addition, the sensor maintained a high performance even at extremely high humidity (90%) and showed outstanding long-term stability. These superb H2S sensing properties were attributed to catalytic sensitization by the Cu dopant and a synergistic effect of the Cu-SnO2 and rGO, which offered abundant active sites for O2 and H2S absorption and accelerated the transfer of electrons/holes.


Introduction
Hydrogen sulfide (H 2 S) is a toxic pollutant gas that degrades the air quality and has negative effects on human health even at low concentrations (10 ppm) 1,2 . On the other hand, H 2 S at the ppb level is also an essential indicator used for diagnoses of diseases, such as diabetes, liver cirrhosis, and asthma [3][4][5] . Therefore, the safety and health of human beings requires highly sensitive monitoring of low H 2 S concentrations.
Colloidal quantum dots (CQDs) are semiconductor nanocrystals with physical dimensions below their Bohr radii, and they are usually synthesized and processed in solution 6 . They present numerous gas-sensing advantages, such as large specific surface areas, porous film structures, and easy integration on virtually any substrate 7,8 . Metal oxide CQDs are commonly used in the design and fabrication of gas sensors. Xu et al. first synthesized SnO 2 QDs for the detection of ethanol using a mixed solvent system composed of oleylamine and oleic acid, but the gas-sensing performance was inevitably hindered by the organics covering the surface 9 . Liu et al. proposed an innovative solution to this problem by utilizing inorganic salts for subsequent surface ligand treatments 8 . This strategy was useful in dealing with relatively large-volume ceramic substrates, yet it was not ideal for micro heating plates with low power consumption. Although various SnO 2 QD/QW-based sensors have been synthesized for H 2 S, NH 3 , and NO 2 detection based on this approach, they still generally suffer from many defects that hinder practical application, such as a strong dependence on ambient humidity, low sensitivity, and a high limit of detection (LOD) 10,11 .
Copper (Cu) is an excellent and sensitive catalyst, and it has recently been dispersed on the surfaces of SnO 2 films to form islands or continuous layers exhibiting selective adsorption of H 2 S 12 . It was found that Cu doping resulted in Cu 2+ occupation of Sn 4+ sites and generated a large number of oxygen vacancies to maintain the charge neutrality, which resulted in enhanced gas-sensing performance of the oxide semiconductor sensors. Additionally, CuO reacts with H 2 S gas to improve the selectivity of the sensor 13 . However, the Cu-doped SnO 2 generally operates at high temperatures (>180°C), which increases the power consumption of the sensor [12][13][14] .
Reduced graphene oxide (rGO)-based layered nanomaterials are two-dimensional carbon materials that have proven to be excellent candidates for decorating MOS gassensing materials and enabling them to work at low temperatures 15,16 . This is because of its high surface area to volume ratio, high charge carrier mobility (200,000 cm 2 /V s at room temperature), active defect sites, and detectable single molecule adsorption/desorption 17 . Furthermore, chemical functionalization with metal and metal oxide nanoparticles allows facile detection of many analytes at low concentrations [18][19][20] . For example, Cui et al. synthesized Indoped SnO 2 /rGO composites via a one-pot hydrothermal method, and they exhibited high selectivities and gas responses at room temperature for NO 2 , with LODs as low as 0.3 ppm 20 . The use of Cu and rGO for SnO 2 sensing, therefore, shows promise for fabricating H 2 S sensors with high sensitivities, low detection limits, and short response/ recovery times. To the best of our knowledge, there have been no reports on the utilization of Cu-doped SnO 2 /rGO for H 2 S sensing.
Herein, the solvothermal method was employed in conjunction with high-temperature annealing to synthesize sensing materials utilizing oleic acid and oleylamine as solvents and surfactants, as well as to investigate their use in H 2 S sensing. The as-synthesized Cu-SnO 2 /rGO had grain sizes of nearly 5.7 nm and surfaces rich in adsorbed oxygen and oxygen vacancies, combined with large BET surface areas and pore sizes. Hence, it exhibited remarkably higher sensitivity (156.5 ppm −1 ) and much lower detection limits (50 ppb) for H 2 S detection than state-of-the-art sensors. A thorough study of the gas-sensing mechanism indicated that the dramatic enhancement in H 2 S sensing performance was mainly dependent on the synergistic effect of the doped Cu and rGO with SnO 2 . This work provides a new perspective for the study of high-performance H 2 S gas sensor fabrication.

Results and discussion
Characterization A schematic diagram for the synthesis of Cu-SnO 2 /rGO is illustrated in Fig. 1. First, the weakly reducing L-ascorbic acid was utilized to reduce the GO. Second, a mixed solvent consisting of oleylamine and oleic acid was employed as a surfactant to control the growth of SnO 2 nanoparticles, and Cu 2+ and the as-synthesized rGO were incorporated as dopants and reacted together. Finally, the Cu-SnO 2 /rGO nanocomposites were prepared via hightemperature annealing (400°C).
In Figs. 2a-c and S1, the TEM and SEM images illustrate the morphologies of the pristine SnO 2 CQDs, Cu-SnO 2 -2, and Cu-SnO 2 /rGO-2 ( Fig. 2a- . The EDS image for Cu-SnO 2 /rGO revealed that the nanocomposites were doped with copper at an atomic ratio of~1%, as shown in Fig. 2h.   The three strongest peaks contained in the XRD patterns of the three samples in Fig. 3a were situated at 26.6°, 34.0°, and 51.8°(2θ), and these corresponded to the (110), (101), and (211) crystallographic facets of the tetragonal rutile SnO 2 structure (JCPDS No. 41-1445), respectively 24 . No additional features associated with CuO/Cu 2 O were observed. Nevertheless, as the amount of Cu doping was increased, the positions of the (100) and (101) peaks for the samples shifted toward lower 2θ values, as shown in Fig. S2a. This indicated that the Cu 2+ replaced some of the Sn 4+ in the SnO 2 lattice to form a solid solution phase, in agreement with the XPS results 25 . No rGO diffraction peaks were observed for the Cu-SnO 2 /rGO nanocomposites (Fig. S2b), probably owing to the low rGO doping level and relatively weak peak intensity indicating that the rGO could not change the lattice structure, consistent with the HRTEM and SAED results. In addition, the characteristic peak intensity for SnO 2 gradually increases with increasing rGO doping, which indicated a continuous increase in the SnO 2 crystallinity. No (002) Bragg peak was observed for rGO.
Raman spectroscopy was utilized to illustrate the reduction of GO and the synthesis of Cu-SnO 2 /rGO. As shown in Fig. 3b, the Raman peaks for the pristine SnO 2 CQDs and Cu-SnO 2 /rGO-2 at 474, 632, and 778 cm −1 corresponded to the E g , A 1g , and B 2g vibrational modes of . The excellent symmetric shapes of these peaks excluded the presence of metallic tin. The disparity in binding energy (0.16 eV) was probably due to the Sn-O interactions resulting after doping with Cu. The high-resolution O 1s spectrum showed peaks for the three samples (Fig. 3e) that were split into three Gaussian peaks and attributed to the three chemical states of O. The O 1s peaks adjacent to 530.5 eV were attributed to the O 2− in the SnO 2 crystal lattice, which is designated lattice oxygen (O lat ) 30 . The peak near 531.0 eV was for oxygen vacancy (O v ), which is attributed to the oxygen-related vacancies in the SnO 2 crystallographic structure 31 . The peak at~532.1 eV was for absorbed oxygen (O ads ), the oxygen species(s) adsorbed by the materials 32 . These three oxygen species are of great importance for gas sensing and will be investigated separately in the Sensing Mechanism section. For Cu-SnO 2 -2 and Cu-SnO 2 /rGO-2, the high-resolution Cu 2p spectrum showed four peaks (Fig. 3f). The peaks at 952.5 and 933.2 eV indicated the Cu 2p 1/2 and Cu 2p 3/2 binding energies, respectively, which confirmed the presence of Cu 2+ and Cu + ions. The two Cu 2p satellite peaks near 962.3 and 942.9 eV corresponded to the CuO phase 14 .
The N 2 sorption−isotherms of the pristine SnO 2 CQDs, Cu-SnO 2 -2, and Cu-SnO 2 /rGO-2 are presented in Fig. 3g-i. The distinct hysteresis loops of the three samples indicated the presence of mesopores 33 . Doping with rGO endowed the Cu-SnO 2 /rGO-2 surface with a larger average pore size (13.1 nm) and increased Brunauer-Emmett-Teller (BET) surface area (90.7 m 2 g −1 ) than the pristine SnO 2 CQDs (11.3 nm; 85.4 m 2 g −1 ) and Cu-SnO 2 -2 (11.6 nm; 82.1 m 2 g −1 ). The larger average pore size facilitated the transport of H 2 S molecules between the ex-and internal regions to enable swift response/recovery even at low temperatures; moreover, the higher BET surface area provided more gas absorption and active sites. Noticeably, the Cudoped SnO 2 showed a slightly lower specific surface area than the pristine SnO 2 CQDs, probably because the originally doped Cu occupied some channels of the SnO 2 . Furthermore, based on UV-vis adsorption spectra, we obtained band gaps of 3.70, 3.65, and 3.56 eV for the pristine SnO 2 CQDs, Cu-SnO 2 -2 and Cu-SnO 2 /rGO-2, respectively, after transformation, as shown in Fig. 4. The narrower bandgap indicated that the electrons in Cu-SnO 2 /rGO-2 transitioned more conveniently from the valence band to the conduction band, which enabled gas sensing and required lower activation energies for chemical reactions 34 .

Gas-sensing performance
The sensing capabilities of the pristine SnO 2 CQDs, Cu-SnO 2 , and Cu-SnO 2 /rGO were systematically evaluated. The real-time resistance was monitored to identify the optimal operating temperature by exposing the SnO 2 -based sensors doped with different concentrations of Cu and rGO to 10 ppm H 2 S at different temperatures, as shown in Fig. 5a. The sensing response (R a /R g ) to 10 ppm H 2 S of the pristine SnO 2 CQDs exhibited a gradual rise as the temperature increased, and the highest response of 4.4 was attained at 280°C. With various Cu doping amounts, the response values all peaked at the same temperature (160°C), and they reached the highest level of 1636.8 for Cu-SnO 2 -2. Furthermore, the rGO dopant reduced the operating temperature down to 120°C, and there was a peak in the sensing response of 1415.7 for Cu-SnO 2 /rGO-2. This was attributed to formation of a p-n heterojunction by the rGO and Cu-SnO 2 , which reduced the activation energy required for the chemical reaction between the semiconductor and the gas molecules. In addition, the underlying mechanism for the reduced operating temperature of the Cu-SnO 2 /rGO-based sensors compared to the pristine SnO 2 CQDs and Cu-SnO 2 can be explained in two ways. First, the rGO exhibited a large specific surface area and a high material submobility, which increased the number of active sites and provided a greater variety of surface adsorbed oxygen species. As shown in Table S1, the contents of O ads in these three materials decreased in the order Cu-SnO 2 /rGO-2 (20.0%), pristine SnO 2 CQDs (10.5%) and Cu-SnO 2 -2 (13.3%), indicating that doping with rGO activated and dissociated O 2 from the ambient air and increased the content of O ads . The increased O ads composition meant that more surface chemisorbed oxygen species were involved in oxidation-reduction reactions, which reduced the activation energy for the reaction between the gas and adsorbed oxygen. Second, the narrower bandgap indicated that the electrons in Cu-SnO 2 /rGO-2 transitioned more readily from the valence band to the conduction band and lowered the activation energy required for the chemical reactions.
Hereafter, 280°C, 160°C, and 120°C were chosen as the optimum operating temperatures for evaluating the H 2 S sensing properties of the pristine SnO 2 CQDs, Cu-SnO 2 -2, and Cu-SnO 2 /rGO-2, respectively. The cross-responses to different gases have been important problems for MOS sensors. To assess the gas selectivity of the pristine SnO 2 CQDs, Cu-SnO 2 -2, and Cu-SnO 2 /rGO-2, these sensors were treated at their operating temperatures with 50 ppm HCHO and C 7 H 8 (toluene) and 100 ppm H 2 , C 4 H 10 (nbutane), and CO. As shown in Fig. 5b, the responses of these sensors to the above gases were all less than 2, much lower than the responses to H 2 S; however, the selectivities of the Cu-SnO 2 -2 and Cu-SnO 2 /rGO-2 were significantly higher than that of the pristine SnO 2 CQDs. The dynamic response-recovery transients for the pristine SnO 2 CQDs (in blue), Cu-SnO 2 -2 (in green), and Cu-SnO 2 /rGO-2 (in orange) after H 2 S exposure/release cycles with different concentrations (1,2,5,10,15,20,30,50, 100, and 150 ppm) are shown in Fig. 5c-e. The responses of the sensors rose sharply with increasing H 2 S concentration. The Cu-SnO 2 / rGO-2 sensor, for which the slope of the linear fit was 156.5 ppm −1 , showed a sensitivity enhanced by over 1900 times in comparison with the pristine SnO 2 CQDs (Fig. 5f). Moreover, the Cu-SnO 2 /rGO-2 sensor featured a linear response (R 2 = 0.991) compared with Cu-SnO 2 -2 (R 2 = 0.921), which tended to become saturated at relatively large concentrations (>50 ppm). In summary, the Cu-SnO 2 / rGO-based sensor presented a better sensing performance than the pristine SnO 2 CQDs and Cu-SnO 2 -2 in terms of operating temperature, linearity, and selectivity. Figure 6 presents the response/recovery curves of the three sensors exposed to 2 ppm H 2 S. The Cu-SnO 2 /rGObased sensor worked at the lowest temperature and exhibited the shortest t res (31 s), which was attributed to the larger specific surface area and the Cu-SnO 2 and rGO heterojunction that reduced the activation energy for gas sensing and accelerated the reaction between H 2 S and the chemisorbed oxygen. In addition, the baseline resistance of Cu-SnO 2 /rGO-2 (~4.5 MΩ) was higher than those of the Cu-SnO 2 -2 (~3.5 MΩ) and pristine SnO 2 CQDs (~0.2 MΩ). This may be due to the ternary heterojunctions that promoted the adsorption and decomposition of O 2 , which formed a higher concentration of chemisorbed oxygen on the surface and resulted in an increase in the thickness of the electric depletion layer. Figure 7a illustrates that the sensor based on Cu-SnO 2 / rGO-2 attained an average of 1.26 for three sequential responses to 50 ppb H 2 S. Hence, we concluded that the LOD of this sensor was less than 50 ppb. As shown in Fig. 7b, the response of the sensor to 20 ppm H 2 S presented similar transients, and all of the resistance values recovered to the initial value for the four consecutive cycles, confirming the outstanding repeatability. Figure 7c displays the behavior of the Cu-SnO 2 /rGO-2 sensor as the relative humidity was varied from 55 to 90%. The responses of the sensor differed slightly as the ambient humidity increased, which indicated that it was only minimally affected by the humidity. In addition, the sensitivity of the Cu-SnO 2 /rGO-2 sensor was almost constant for 28 days, as demonstrated in Fig. 7d, and this indicated its good long-term stability. In Table S2, we have summarized the performance of the Cu-SnO 2 /rGO sensor and compared it with those of other SnO 2 -based H 2 S sensors reported in the recent literature for the sake of comparison. The Cu-SnO 2 /rGO-2 sensor prepared in this work showed high sensitivity, a low detection limit, and fast recovery at relatively low operating temperatures, which indicated that the prepared Cu-SnO 2 /rGO-2 sensor has broad development prospects and potential for use in H 2 S detection.

Sensing mechanism
The sensing mechanism of the SnO 2 (an n-type MOS) sensor involved gas adsorption and surface-related redox reactions 35 . A schematic of the gas-sensing mechanism is provided in Fig. 8a. For the Cu-SnO 2 /rGO synthesized in this work, oxygen molecules from the ambient atmosphere were adsorbed on the surface of the material and converted to oxygen anions by trapping electrons from the conduction band. Under the test conditions (120°C), the surface oxygen species were primarily O 2− and O − 36 . The loss of electrons caused the formation of an electron depletion layer in the surface region, while a potential barrier was built between the adjacent grains; this impeded the flow of electrons at the grain boundaries, which manifested itself macroscopically as an increased resistance. When the sensor was exposed to H 2 S, oxygen ions reacted with H 2 S and delivered electrons to the Cu-SnO 2 /rGO surface. As a result, the electron depletion layer narrowed, the barrier between the adjacent grains was reduced, and the resistance decreased. The exact reactions are shown in ref. 37 .
The sensor based on Cu-SnO 2 /rGO operated at the lowest temperature and showed higher sensitivity than the pristine SnO 2 CQDs and Cu-SnO 2 for the following reasons. First, Cu-SnO 2 /rGO formed a special p-n-p ternary heterostructure. The energy band structure is shown in Fig. 8b. Under an ambient atmosphere, electrons were transferred from SnO 2 to the rGO and CuO, whereas holes were transferred in the opposite direction until the Fermi energy level reaches equilibrium. The electron depletion layer at the heterojunction interface was hence wider than that of pristine SnO 2 , corresponding to an increase in the baseline resistance. H 2 S reacted with the surface negative oxygen species, and electrons entered the conduction band of SnO 2 . Additionally, some electrons also entered the conduction band of rGO and CuO, which was manifested at the macroscopic level as an enhanced conductivity. In addition, the heterojunctions also contributed to the catalytic activity by providing more adsorption reaction sites 38 . Second, according to the three oxygen species occupancy ratios derived from the O 1s XPS data (Table S1), the Cu-SnO 2 /rGO surface had more adsorbed oxygen and oxygen vacancies. The abundant chemisorbed oxygen promoted adsorption and reaction of the reduced gas; the oxygen vacancies contributed to the increasing charge density near the valence and conduction bands, which narrowed the bandgap of SnO 2 (Fig. 4) and facilitated adsorption and activation of the target gas. Moreover, the mesoporous structure provided effective diffusion channels for the gases, and the larger BET surface area provided more active sites for foreign oxygen molecules, which enabled penetration of the gaseous molecules and interactions with the interior grains. Finally, the Cu-SnO 2 /rGO grain size (5.7 nm) was the smallest and close to 2L D (3 nm at 120°C), which maximized the effect of varying the electron depletion layer thickness on the overall resistance.

Conclusions
In summary, a Cu-doped SnO 2 /rGO-based H 2 S gas sensor was successfully synthesized via the solvothermal method. Compared to the pristine SnO 2 CQDs and Cu-SnO 2 , the gas-sensing performance of the Cu-SnO 2 /rGO sensor was remarkably improved, with an ultrahigh sensitivity (156.6 ppm −1 ), an ultralow detection limit of 50 a Schematic diagram of the gas-sensing mechanism for the sensor based on Cu-SnO 2 /rGO. b Energy band structure for the p-n-p heterojunction of Cu-SnO 2 /rGO. The band structure data in (b) were determined from the literature 40,41 ppb (R a /R g = 1.26), and a rapid response time (31 s, 2 ppm). In addition, the sensor operated effectively at high humidity (90%). These excellent H 2 S sensing properties were attributed to the synergistic effect of Cu and rGO with the SnO 2 : the smaller grain sizes, larger specific surface area, unique p-n-p heterostructure, increased oxygen vacancies, and narrower band gap structure increased the sensitivity of the sensor, and the larger pore size provided shorter response/recovery times for the sensor. Thus, these Cu-SnO 2 /rGO ternary nanocomposite sensors are promising candidates for fast, highly sensitive, and low-concentration detection of H 2 S. Additionally, we found that high-temperature annealing effectively reduced the organic coverage on the surface of SnO 2 CQDs, which improved the gas-sensitive performance and reduced the influence of ambient humidity. We will study the mechanism of high-temperature annealing in future work.

Synthesis of rGO
GO was reduced by utilizing the green agent AA in a 95°C water bath. In a typical procedure, GO (25 mg) was dispersed in deionized water (25 ml) and sonicated for 1 h to prepare a homogeneous GO dispersion (1 mg/ml). AA (250 mg) was then added to the GO dispersion and maintained at 95°C in a constant temperature water bath for 24 h. After cooling to room temperature, the dispersion was rinsed 2-4 times with ethanol and deionized water to remove impurities. Finally, the rGO was dried in an oven at 75°C overnight to obtain the rGO solid powder for characterization.

Synthesis of Cu-SnO 2 /rGO nanocomposites
We synthesized the SnO 2 CQDs via a slightly modified version of the solvothermal process reported by Xu et al. 9 . In a typical process, SnCl 4 (1.2 mmol) and CuCl 2 (0.6 mmol) were distributed in oleic acid (20 ml) and oleylamine (2.5 ml) by sonication for 10 min, followed by vigorous stirring at 60°C to form a transparent solution. Subsequently, a rGO ethanol dispersion (3.6 ml, 1 mg/l) and ethanol (6.4 ml) were added in turn and stirred to achieve a hyaline. The solution was then transferred to a 50 ml Teflon-lined autoclave and maintained at 180°C for 12 h. After natural cooling, the solution was washed with ethanol and hexane several times and then dried at 75°C overnight. The as-synthesized powders were then calcined in a muffle furnace at 400°C for 2 h with a 10°C min −1 heating rate. The total molar ratio of SnCl 4 to CuCl 2 was kept constant, while varying the molar ratio of Cu 2+ to Sn 4+ yielded pristine SnO 2 CQDs, Cu-SnO 2 -1 (1:4), Cu-SnO 2 -2 (1:2), and Cu-SnO 2 -3 (1:1). A series of Cu-SnO 2 / rGO nanocomposites were obtained with the indicated volumes of the rGO ethanol dispersions; Cu-SnO 2 /rGO-1 (1.8 ml), Cu-SnO 2 /rGO-2 (3.6 ml) and Cu-SnO 2 /rGO-3 (5.4 ml). A schematic diagram for the synthesis of Cu-SnO 2 /rGO is provided in Fig. 1.

Characterization
The sizes and morphologies of the products were obtained by transmission electron microscopy and highresolution transmission electron microscopy (TEM and HRTEM, FEI Tecnai G2 F30) with an energy-dispersive X-ray spectrometer (EDS, Xplore) operating at an accelerating voltage of 300 kV. The phase purities of the nanocrystals were determined with powder X-ray diffraction (XRD, Bruker D8) operating at 40 kV and 40 mA with Cu Kα irradiation (λ = 1.5406 Å). Scans were taken with a 2θ range of 20°-80°and step sizes of 6°min −1 . Raman spectroscopy was performed with a HORIBA Scientific LabRAM HR Evolution instrument with the 514 nm line of an Ar + -ion laser. The surface compositions and bonding states of the nanocrystals were obtained by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) with an aluminum source; all binding energies were referenced to the C 1s peak at 284.8 eV for surface carbon. The specific surface areas and porosities of the as-synthesized samples were determined from nitrogen adsorption-desorption isotherms (Micromeritics TriStar III 3020) generated at 77 K. Ultraviolet-visible (UV-vis) absorption spectra were measured with a Perkin-Elmer Model Lambda 950 UV-vis/NIR spectrophotometer.

Gas-sensing measurements
The gas-sensing method was described in detail in our previous work 39 . Briefly, the as-synthesized samples were first well ground with agate and then mixed with ethanol to form a 15 mg/ml suspension. This was then coated on the micro thermal plate and dried at 60°C for 2 h. For strong adsorption of the H 2 S, the purity of the gas was calculated with the stationary-state gas distribution method in this work. The desired concentrations of H 2 S (C) were obtained by diluting the standard H 2 S gas (100 ppm) with air as the background gas and calculated as C = V s × C s /V, where V s is the volume of standard gas that was injected into the chamber, C s is the concentration of the standard gas (100 ppm standard gas mixed with clean air), and V is the volume of the sealed chamber (1 l). All measurements were carried out at~55% RH and 25°C, except for those determining the effect of humidity.
The sensor response S was calculated as S = R a /R g , where R g and R a are the resistance in the target gas and air, respectively. The response and recovery times (t res and t rec ) were defined as the time for the sensor to reach 90% of the total change in resistance. Moreover, the sensitivity of the sensor was expressed as the change in the measured response signal per ppm unit, i.e., the slope of the linearly fitted response line after calibration.